Chapter 1
Cutting-Tool Materials
(continued)

1.3.3 Coated Carbide Tools

While coated carbides have been in existence since the late '60s, they did not reach their full potential until the mid 70s. The first coated carbides were nothing more than standard carbide grades which were subjected to a coating process. As the manufacturers gained experience in producing coated carbides, they began to realize that the coating was only as good as the base carbide under the coating (known as the substrate).

It is advisable to consider coated carbides for most applications. When the properly coated carbide with the right edge preparation is used in the right application, it will generally outperform any uncoated grade.

Numerous types of coating materials are used, each for a specific application. It is important to observe the do's and dont's in the application of coated carbides. The most common coating materials are titanium carbide, titanium nitride, ceramic coating, diamond coating and titanium carbo-nitride.

In general the coating process is accomplished by chemical vapor deposition (CVD). The substrate is placed in an environmentally controlled chamber having an elevated temperature. The coating material is then introduced into the chamber as a chemical vapor. The coating material is drawn to and deposited on the substrate by a magnetic field around the substrate. It takes many hours in the chamber to achieve a coating of 0.0002´´ to 0.0003´´ on the substrate. Another process is Physical Vapor Deposition (PVD).

Of all the coatings, titanium carbide is the most widely used. Titanium carbide is used on many different substrate materials for cutting various materials under varying conditions. Titanium carbide coatings allow the use of higher cutting speeds because of their greater resistance to abrasive wear and cratering and higher heat resistance.

There are a few important points to remember about using coated carbides. Coated carbides will not always out-perform uncoated grades but because of the benefits offered by coated carbides, they should always be a first consideration when selecting cutting tools.

1.4 Ceramic and Cermet Tools

Ceramic aluminum oxide (Al2O3) material for cutting tools was first developed in Germany sometime around 1940. Cermets are basically a combination of ceramic and titanium carbide. The word cermet is derived from the words "ceramic" and "metal."

Ceramic Cutting Tools: Ceramics are nonmetallic materials. This puts them in an entirely different category than HSS and carbide tool materials. The use of ceramics as cutting tool material has distinct advantages and disadvantages. The application of ceramic cutting tools is limited because of their extreme brittleness. The transverse rupture strength (TRS) is very low. This means that they will fracture more easily when making heavy or interrupted cuts. However, the strength of ceramics under compression is much higher than HSS and carbide tools.

There are two basic types of ceramic material: hot-pressed and cold-pressed. In hot-pressed ceramics, usually black or gray in color, the aluminum oxide grains are pressed together under extremely high pressure and at a very high temperature to form a billet. The billet is then cut to insert size. With cold-pressed ceramics, usually white in color, the aluminum oxide grains are pressed together, again under extremely high pressure but at a lower temperature.

Hot- and cold-pressed ceramic inserts come in many sizes and shapes.

While ceramics may not be the all-around tool for the average shop, they can be useful in certain applications. Ceramic tools have been alloyed with zirconium (about 15%) to increase their strength. Many ceramic tool manufacturers are recommending the use of ceramic tools for both rough cutting and finishing operations. Practical shop experience indicates that these recommendations are somewhat optimistic. To use ceramic tools successfully, insert shape, work material condition, machine tool capability, set-up, and general machining conditions must all be correct. High rigidity of the machine tool and set-up is also important for the application of ceramic tools. Ceramics are being developed to have greater strength (higher TRS).

Cermet Cutting Tools: The manufacturing process for cermets is similar to the process used for hot pressed ceramics. The materials, approximately 70 percent ceramic and 30 percent titanium carbide, are pressed into billets under extremely high pressure and temperature. After sintering, the billets are sliced to the desired tool shapes. Subsequent grinding operations for final size and edge preparation, complete the manufacturing process.

The strength of cermets is greater than that of hot pressed ceramics. Therefore, cermets perform better on interrupted cuts. However, when compared to solid ceramics, the presence of the 30 percent titanium carbide in cermets decreases the hot hardness and resistance to abrasive wear. The hot hardness and resistance to abrasive wear of cermets are high when compared to HSS and carbide tools.

Silicon-Nitride Base Ceramics: Developed in the '70s, silicon-nitride (SIN) base ceramic tool materials consist of silicon nitride with various additions of aluminum oxide, yttrium oxide, and titanium carbide. These tools have high toughness, hot hardness and good thermal shock resistance. Sialon, for example, is recommended for machining cast irons and nickel base superalloys at intermediate cutting speeds.

1.5 Diamond, CBN and Whisker- Reinforced Tools

The materials described here are not commonly found in a heavy metal working environment. They are most commonly used in high-speed automatic production systems for light finishing of precision surfaces. To complete the inventory of tool materials, it is important to note the characteristics and general applications of these specialty materials.

This photo from Sandvik Coromant Co. shows polycrystalline diamond material bonded to a carbide base.

Diamond: The two types of diamonds being used as cutting tools are industrial grade natural diamonds, and synthetic polycrystalline diamonds. Because diamonds are pure carbon, they have an affinity for the carbon of ferrous metals. Therefore, they can only be used on nonferrous metals.

Cubic Boron Nitride: Cubic boron nitride (CBN) is similar to diamond in its polycrystalline structure and is also bonded to a carbide base. With the exception of titanium, or titanium-alloyed materials, CBN will work effectively as a cutting tool on most common work materials. However, the use of CBN should be reserved for very hard and difficult-to-machine materials. CBN will run at lower speeds, around 600 SFPM, and will take heavier cuts with higher lead angles than diamond. Still, CBN should mainly be considered as a finishing tool material because of its extreme hardness and brittleness. Machine tool and set-up rigidity for CBN, as with diamond, is critical.

Whisker-Reinforced Materials: To further improve the performance and wear resistance of cutting tools to machine new work materials and composites, whisker-reinforced composite cutting tool materials have been developed. Whisker-reinforced materials include silicon-nitride base tools and aluminum-oxide base tools, reinforced with silicon-carbide (SiC) whiskers. Such tools are effective in machining composites and nonferrous materials, but are not suitable for machining irons and steels.T&P

The full chapter, with all accompanying illustrations, can be found on the T&P web site http://www.toolingandproduction.com.

Originally published in the January 2001 issue
of Tooling & Production.
Please Note: some pictures or diagrams are only available through the printed media.